AMC 12 2004b Problem 3

If $x$ and $y$ are positive integers for which $2^x3^y=1296$, what is the value of $x+y$?

$(\mathrm {A}) 8 (\mathrm {B}) 9 (\mathrm {C}) 10 (\mathrm {D}) 11 (\mathrm {E}) 12$